The Great Lakes system is the largest source (approximately 22%) of available fresh surface water in the world, and Lake Michigan is the second largest of the Great Lakes by volume. Northeastern Illinois borders the shores of this Great Lake, which is the largest public drinking water supply in the state, serving approximately 6.6 million residents.[1]  However, Illinois’ use of Lake Michigan water is limited. Since the reversal of the Calumet and Chicago Rivers away from Lake Michigan, most of Illinois is no longer in the Great Lakes watershed. The water we pump out of the lake now flows west and south and ultimately to the Gulf of Mexico.

Due to this diversion of water out of the Great Lakes, the State of Illinois is under a U.S. Supreme Court Decree that limits the amount of water it can use. Communities in Illinois that seek to use Lake Michigan as a drinking water source must apply to the Illinois Department of Natural Resources (IDNR), which is the state agency that governs permits for Lake Michigan water and oversees the state’s compliance with the decree. Recent Illinois usage of its water allocation from Lake Michigan has reached about 76 percent of the allowed total.[2] While we may think our region enjoys an unlimited amount of freshwater, that is not true.

Groundwater is underground, held in soil or in the pores and crevices of rock. Most communities around the world get their drinking water from beneath the earth’s surface. Here in Illinois, there are two main types of aquifer: shallow and deep. Shallow aquifers lie within 500 feet of land surface, while deep aquifers are located at depths of at least 500 feet below the surface. Deep bedrock aquifers supply the majority of drinking water for groundwater-dependent communities in Northeastern Illinois. 

Both shallow and deep aquifers have different challenges that need to be addressed. The more minerals—such as saline water, barium, radium or other elements—that need to be removed from a groundwater source, the higher the treatment costs.

Groundwater recharge is an important part of the hydrologic cycle. When there’s precipitation, water moves through the surface of the earth to natural storage areas below the ground, which “recharge,” or refill, these water sources. The more impervious the land cover—such as buildings, pavement and concrete—the less water reaches these water sources to recharge. A deep bedrock aquifer can take many decades to recharge through rain and snowmelt. While shallow limestone, or sand and gravel aquifers in Northeastern Illinois recharge faster than deep bedrock aquifers, they are more susceptible to contamination and can be depleted faster when used for drinking water. And shallow aquifers can impact surface water and wetlands since they are interconnected and often exist directly above or near these resources. 

The depletion of aquifers reduces both water supply availability and quality. The consequences of over-pumping water include potential contamination and increased energy usage, both of which lead to increased treatment costs, before, eventually, wells become unusable.